Being able to "just let go" while laying down.

This is DAMN NEAR IMPOSSIBLE. For the life of me, I cannot wet while laying down on my back or stomach. I know it isn't impossible though, because I've done it when I used to wear diapers around the age of 10, and I used to be able to "let go" easier. I'm not talking about forcing it, because everyone knows that is possible, but it's not comfortable.

I need some advice it seems. I don't think this is physically possible anymore, somehow...

I have no trouble letting go while lying on my back any more. However, that took months if not a whole year of practice, so don't worry if you can't manage to do it after one or two nights.

Your body/brain has learned to only let go in certain positions (for guys mostly standing), so you have to do no less than undoing potty training, which isn't easy. Take it step by step and learn how to wet your diaper in a sitting position. Once you've mastered that, you can just sit up in your bed and wet your diaper, or prop up your body with your arms. Over time, you'll find that you won't even have to sit up any more in bed.
I'll also help if you teach yourself to pee in unnormal positions when not wearing a diaper. I.e. sit down on the toilet to pee, pee into an old bottle while sitting at your desk or whatever else. Basically, anything that'll teach your mind that peeing can be done outside of the bathroom and while not having a toilet in your field of vision. Remember - you're physically capable of peeing while lying down, it's "only" a mental block!

Even I haven't be able to do it easily yet...You just need to practice some...I'd try on your stomach first, because I believe it's easier...The only problem with that, is that it's more prone to leaking on your stomach

It just takes time and practice to relax yourself and get into the proper frame of mind for it. I can do it, but since there's a much higher risk of leaks, I generally don't.

If you simply must, I found that it was helpful to be at a point where I was able to go, but short of the point where I really needed to go. Being full up sort of raised my stress level and gave me a harder time relaxing, which for me, was the most important bit. If you can put your mind mostly elsewhere and just give a little push now and again you might get there. If you're really desperate, stand up, go just a bit and stop. Once the pump is primed (so to speak), you will likely have an easier time.

It takes time. A lot of time. I am relatively inexperienced myself and while I can get stuff out it takes ages for me to go and it is never that much, which I suppose is good, it stops me flooding. As I am in a similar situation to you I'll tell you what I try to do.

Firstly I try to be relaxed. Been as I am in bed when lying down so probably trying to go to sleep this isn't much of an issue. In particular try to feel your bladder, and focus on relaxing it. If you are lucky something might happen. Other option is to try and push it out. If you build up enough pressure eventually it just goes.

Just one warning. Don't try going on your stomach. That is just asking for a leak and it is better not to have to clean up a leak. You are trying to encourage your brain to let you go lying down. If you have to clean up leakage your brain could build a negative association and be even harder to convince.

Not that I had the issue for long, but I would say that shen you fell like you need to get up and use it, stand up, but as soon as you start to go, lay back down, this will get your body in the habbit of going in the final position.